INTERACTION OF POLY(ALPHA,BETA-ASPARTATE) WITH OCTADECYLAMINE MONOLAYERS - ADSORPTION BEHAVIOR AND EFFECTS ON CACO3 CRYSTALLIZATION

Langmuir monolayers have been used to study the interfacial adsorption of poly(alpha,beta-aspartic acid) peptides. At pH 5-6, isotherms for the positively-charged octadecylamine (CH3(CH2)(16)NH3+) monolayer sho wed expanded Limiting areas per molecule (30-35 Angstrom(2)) compared to octadecylamine spread on pure water (approximate to 20 Angstrom(2)) , suggesting strong adsorption and partial intercalation of a 4.1 kDa peptide between surfactant molecules. Branched-chain peptides of diffe rent molecular masses (2 and 5 kDa) and a linear 7.1 kDa molecule had similar effects, but the alpha-aspartate monomer had no influence on t he isotherms. Thus, polycarboxylate molecules were required for effect ive adsorption although the number of acidic groups on the polymers wa s not important. The presence of calcium ions in the subphase, at pH 5 -6, appeared to have little effect for the branched peptides studied, but the linear 7.1 kDa poly(alpha,beta-aspartate) showed enhanced, if variable, intercalation (30-75 Angstrom(2)). This was probably a resul t of conformational changes rendering the molecule more predisposed fo r adsorption. No effects were observed with negativelycharged or neutr al monolayers, indicating that the driving force for adsorption at the air/water interface was electrostatic. The presence of poly(alpha,bet a-aspartate) in supersaturated calcium bicarbonate solutions resulted in vaterite nucleation rather than calcite crystallization as in contr ol experiments. Calcium carbonate crystallization under compressed oct adecylamine monolayers produced oriented vaterite crystals, whereas in the presence of the peptides, nonoriented vaterite aggregates were fo rmed at the monolayer/solution interface.